'75 50 Watt Mkii ...interesting Beast!

[myersbw]

Ok, I've had a variety of 60's-80's Marshalls across my bench this past, yet still short 4 years. But, this one was intriguing in a most unusual way and my time with it was cut short. It came in the door with two goals in mind by the client...bias a new set of tubes for it and try to fix the issue of near max volume between 1 & 2 on the dial.

The bias was typical and quick. But, it was shrill. Clipped the bright cap, but then there's this "fast gain thing"...and, there's more...you couldn't reduce the volume to zero with the amp's volume control. Hmmm.

Essentially, this amp.... http://drtube.com/schematics/marshall/1987u.gif

The last issue bothered me more than anything. There was about 25 ohms on the wiper-to-pot reading, but not real significant compared to 1,000,000. Still, lots of play in the shaft, so I installed a new one. No change. Hmmm.

To Google with a search which led to a year old thread in here about a different '74-75 model with similar issues...very hot amp and someone's wouldn't turn down volume completely. Statements of normal, but I don't subscribe to that thought. Well, might be common...shouldn't be what to expect.

I had a little time left. So...I was curious about the source of the no-turn-off. Bright cap was already off. I disconnected the volume control's wiper lead and grounded it to chassis. Now it's LOUDER and clean with a little breakup when played hard. (Obviously some phase shifting as the induced signal now increased) Huh? Ok...next I took a jumper and shorted the pot to ground...effectively grounding the ac signal from the cathode of V1b. And, but the way, made no difference if the V2a grid was grounded or not...did not change the level.

Hmmm...tone stack? Yep...affected the signal. And, then I ran out of time to test anything else, by scope or whatever. Still needed to tame the volume control range. The easiest way to restore it was pop a 470K resistor across the control's 1 & 3 lugs. In addition, I added a series 470K inline with the wiper lug and V2a's grid lead (no time to pull a board/controls to replace the current 470K mixer with a 1M).

The result was amazing. It started with being an amp that I really didn't think much of to one that had not only so much growl, the cleans were the most amazing I've heard a Marshall produce...fat, so full and just a hint of chime (yet tight) and a lot of sparkle on the top end, but not excessive. All for just a very subtle mod.

Some folks have hinted at induction via close components. I'm wondering if the culprit might be that power cap under the board with some excessive ESR at this point in it's life. The client wasn't willing to pay for a cap job yet, but if I see that amp again...I'll have to explore it a bit.

But, now it has me wondering how many more out there have a similar issue. Especially in light of what I'll bet many of us do...when we stop playing, the first thing we do is turn the guitar's volume down...not the amp. (So, you might not know your amp has that symptom.)

Anyone else have a similar '75 or '76 MKii that they've tamed and found the source of the induced volume? Given the audio level, I'm just guessing, but I'd say no more than maybe a 1/4 to 1/2 watt? Also...zeroing all the other pots made no difference. I'd have to go with back up through the power chain...? End result...client took the amp back and loves it. I'm not surprised...the tone was awesome in spit of the quirks.

Brad

 

[neikeel]

Yes - on a 76 JMP50 (1987 Lead)
It pretty much did the same, but the normal channel was very weak in comparison.
It was a mean rock and roll amp and the owner did not want it messing with - just checked over and I did not have time to poke too much, I suspect some signal bleed (relatively one way because of the bypass caps on the lead channel)
Interesting tho'

 

[liontato]

Pretty cool!

 

[myersbw]

I'd have loved to had time to replace that end of chain power cap and note any differences.

Yes - on a 76 JMP50 (1987 Lead)
It pretty much did the same, but the normal channel was very weak in comparison.
It was a mean rock and roll amp and the owner did not want it messing with - just checked over and I did not have time to poke too much, I suspect some signal bleed (relatively one way because of the bypass caps on the lead channel)
Interesting tho'

 

[mickeydg5]

So did the potentiometer have a good AC connection to chassis ground?

 

[myersbw]

Yes, it did...and does. The induced signal is out of phase as reconnecting that stage's pot and restoring the wiper connection greatly reduced the induced signal, but it was still there. The imposed signal when you turn up the level on that stage just overpowers it and it makes no difference.

Trust me...I measured points to ground everywhere. I did have a couple jack ground issues due to oxidation, but DeOxit on a soaked Q-tip took care of that. Really wanted more time with that one. But, the client is totally happy with how it performs with minimal cash outlay on his part. (I don't charge for research I do and they don't ask for.)

As for the amp turned up...I really liked it.

 

[mickeydg5]

I figured you probably had checked things. You are a thorough guy. I just thought I would ask.
It is a peculiar scenario.

 

[myersbw]

Well, the fella is a steady client and I think, of a dozen vintage Marshalls, he very well will keep that one. Lol, I'll see it again!

 

[Chrome]

I have the 76 version of MKII 50 watt, I love it. but I had to install PPMIV for my use, I play mostly hard southern rock and couldn't get the growl I wanted without bleeding ears. now I am totally satisfied with it.

 

[myersbw]

Hey Chrome! If you're doing your own work...try the resistor mod on Ch 1. All done at the pot for easy reversal...take you 5 min. Pop a 470K in parallel with the volume pot (terminals 1 & 3...the old cheater method of reducing overall pot value) and pull the wiper lead, put another 470K on the wiper terminal and then reconnect the wiper lead to the resistor. Just changing the voltage divider really to reduce the control swing early on and still have a screaming amp. I'm curious to what you might think of the clean range at this point and if you still retain enough to push those output tubes as you wish. For the one on my bench, it changed tolerable output for clean to the 5-6 position (vs. 1-2) and it had no PPIMV no time to test the waters for anything else. But, if you're relying on PPIMV, you're really just capitalizing on an extra tube's gain with clipping the PI vs. full throttle power use clip anyways.

Let me know if you try it...wouldn't take much. It produced one of the lushest clean tones I've heard from a Marshall yet, for me.

I have the 76 version of MKII 50 watt, I love it. but I had to install PPMIV for my use, I play mostly hard southern rock and couldn't get the growl I wanted without bleeding ears. now I am totally satisfied with it.

 

[myersbw]

By the way, this particular 50w Mk ii had linear volume pots. The replacements I used were audio taper and I would've thought that might be sufficient to restore the signal taper to acceptable...wasn't even close. Hence the two resistor mod. I did this as a ballpark starting point (and it was fast to do), and liked it so much I left it.

I'm wondering what Marshall's push was for this design? Provide the "Wow, man, look how much gain this dude has! I'm just on 1!!! Must be a monster! (Exclaimed while puffing on the preferred illegal substance of the day)" lol. Or, was it, as someone else mentioned somewhere, Marshall had such an increase of demand that they took whatever pots they could get...linear, log...anything to get the amps out the door. ?

Thoughts? Or, was anyone here working for them in the day that might know?

 

[myersbw]

Just heard back from client. He pushed the amp to full. As i suspected, there was plenty of signal to push the power tubes over the top and get a nice creamy crunch. So there ya go, a non-invasive, fast, reversible TRM (two resistor mod) to tame those hungry early 70's Mk ii's out there. Lol

Given the changes, I'm curious to how complete the mod would be by just replacing that pot with a 250K audio taper and include the series 470K off the wiper. Might be just the right stock part change. The resistor in parallel parks the pot's overall impedance effect at 320K in & of itself. Dropping to 250K should be fine and give you a little more swing toward full throttle?

I certainly need more time to play with amps when they're in the door.

 

[Chrome]

I may try that, my confidence in doing mods is minimal. I done the ppimv mod out of necessity. I am a 35 year veteran controls guy but not a lot of knowledge in the electronic department and this amp is a substantial investment to me. the question I have is I currently have channel jumper in. I was thinking about trying an AB switch using one channel for cleaner and the other for gain? anyone else do this? your thoughts?

 

[myersbw]

I do know some folks in the area that do use an external A/B to channel switch and they're happy with it. What I've suggested is far less invasive than a PPIMV since you've done that. I've played with "all and including the kitchen sink" mods and just find that I tend to minimize what I do unless someone really insists on paying for and going an extra mile.

I make an effort to preserve reversing everything with no ill effect on value. Since you have the PPIMV, my guess is you'd really benefit from just adding these two resistors (and, btw, the bright cap is clipped out in this process, too). I probably wouldn't change the pot if it's fine. What I've done takes literally 5 minutes once the chassis is out.

And, for that particular amp...I wouldn't do anything extra. The only real significant tonal change comes from the bright cap removal...the resistors are simply to give your volume control knob a much more manageable range. I could've tweaked the values to fine tune it, but lack of time coupled with..."it just works well now"...forced me to stop.

 

[Yugedrums]

...aaaaand I wish you lived near me. (No techs around here).

I don't understand a lot of what you said, but I love what I did understand. My 1959 reissue did some of this stuff when I got it. On the high treble channel, there was a little sound heard when the volume was all the way on zero and the volume went from zero to "blow-the-windows-out" from 1 to 1.5 on the knob. It also had a horrible, ice-pick shrill. I had to jumper the channels and blend them (like an EQ) just to get a decent sound out of it, which worked fine and is what many people do anyway.

I ended up reading about the caps at C17 and C18 (4n7, 1n0), so I removed them and it eliminated all of that. Apparently one is the bright cap and the other is the "full-bast-at-1.5" cap. The bright channel volume rises normally now and sounds normal (no ice pick). I was then able to use this channel by itself, with no jumpering.

I eventually installed a LarMar PPIMV and the amp became my favorite Marshall I had ever owned. Crank the preamp and adjust volume with the master and it's like one of my 800's (a little more gain than my 800's, actually). Turn the master all the way up and adjust volume with the preamp and it's exactly the same Plexi it was before. The best of both worlds and it does both things VERY well. It sounds fantastic.

I do fine with removal/replacement, etc and can solder well (been doing it for decades)... but I wish I knew what some of you guys know and could get all inside the actual theory of this stuff. Please keep these kinds of posts/threads coming, I've learned TONS just reading these! It's an invaluable resource.